Electrical switching apparatus



Aug. 18, 1959 M. P. REECE 2,900,476

ELECTRICAL SWITCHING APPARATUS Filed April 8, 1957 United States Patent ELECTRICAL SWITCHING APPARATUS Michael Peter Reece, Greenford, England, assignor to E. R. A. Patents Limited, Leather-head, England, a British company Application April 8, 1957, Serial No. 651,280

Claims priority, application Great Britain April 17, 1956 8 Claims. (Cl. 200-144) This invention relates to vacuum switches for alternating current operation and is particularly concerned with such switches suitable for use as contactors in which the contacts have to open and close very frequently.

In known vacuum switches, the contacts, for example of copper (tungsten and steel are also employed), are formed initially with operating faces which although theoretically flat or domed are actually in contact with one another at a few very small points. As a consequence, there is a high current density at these points, and as the contacts are separated, with the production between them of arcs in the copper vapor emitted from the cathode spots, molten bridges of copper are drawn out between the contacts at these points. As the contacts completely separate, these bridges explode with a resultant loss of the copper of the contacts, and this loss, together with the loss produced by evaporation of the copper from the cathode spots by the heat generated by the arcs during operation of the switch, causes a continual erosion of the whole of the operating faces of the contacts. Such erosion may amount to one twentieth of an inch for a million operations at a hundred amperes R.M.S. for copper contacts of half inch diameter. This erosion, which is considerably less than that found in airbreak types of switch, would not be so serious in these types of switch, where the movement of the contacts on separation is measured in inches, but is serious and becomes progressively more so with time, for vacuum switches, where the movement is only a millimeter or so. Any increase of this movement to take up the effects of erosion involves adjustments of the switch and also involves excessive movements, and thus risk of fatigue, of the metal bellows which is provided to preserve the vacuum while allowing the movements of the contacts to take place.

Known vacuum switches are, in fact, limited, by the erosion referred to, to a life of only a few million operations, and they are therefore hardly better than airbreak types of switch. In addition, current chopping is liable to occur in these known switches even when they are designed for, and used at, a particular voltage, other than the very highest voltages. Thus the interruption of a small current in a 200 volt inductive circuit by a vacuum switch with copper contacts may lead to over-voltages greater than 50,000 volts and these would break down the circuit.

According to the present invention, each contact comprises a mechanically strong conducting body having in its operating face one or more recesses containing a nonfluid material (i.e; a material which is not fluid when the switch is inoperative) which has a lower arcing voltage than that of the body and which is capable of constituting an adherent conducting film on the operating faces during operations of the switch. In practice, the operating faces would be exposed when the contacts are new, so that these faces come into direct engagement with one another as the contacts are brought together, but during operations of the switch these faces become covered with a film of the material (referred to as the active material) 2 in the recesses. This covering process takes place as follows, assuming that the contacts are new.

When the contacts are separated, it has been found that the cathode spot of the arc drawn between their operating faces splits up into many spots (each with the arcing voltage of the body) and these spots very quickly move about the surfaces of the contacts until one or more of these spots reach the active material in the recesses. When this happens, the arcing voltage of these particular spots falls to the arcing voltage of the active material, while the remaining spots die out. Thus all the current is transferred to the spots of lower arcing voltage (i.e. those on the active material), and these spots split up and quickly spread over the surface of this active material. As a result, the material in the immediate neighborhood of the spots liquifies and evaporates and most is transferred from the recess or recesses containing the cathode spots into a recess in the other contact. Because the switch is employed to control alternating current, during its many operations as much active material is transferred in one direction from one contact to the other as in the opposite direction, so that little active material is lost from the recesses. Some of the active material, however, moves away from the recesses and is deposited on to the operating faces of the body of the contacts to form the film, while a very small amount is lost completely by passing out of the region between the contacts and collecting on the walls of the envelope in which the switch operates. During opening of the switch, bridges of active material form between the film of active material on one contact and that on the other, but a smaller current density is needed to form and explode these bridges than would be necessary in a switch with plain contacts made of the same material as the mechanically strong body, and this density is insufiicient to cause melting of the body of either contact. It will be realised that this density is the same as is needed in a switch with plain contacts made of the active material, but in such a switch erosion of the whole of the operating faces takes place, as previously mentioned.

The film of active material increases in thickness until it reaches a stable value (of the order of 0.01 mm.) at which as much active material is lost from the operating faces (being deposited on the envelope walls or returning to the recesses) as is gained from the material in the recesses. If the film becomes thinner than this stable value for any reason, the cathode spots of the are are found to move very quickly into the region of the recesses and then remain there so that more active material is evaporated from these recesses: this material is then deposited on the operating faces and increases the thickness of the film. If, on the other hand, the film becomes thicker than the stable value, the cathode spots are found to move more slowly over the contacts so that more of the active material is evaporated from the operating faces and less from the recesses. The attainment of a stable thickness, with no erosion of the body of either contact, ensures, of course, that the same relative movement of the contacts will bring about opening and closing of the switch for very many millions of operations: for example, one contactor has been constructed according to the invention capable of performing 10 operations, which is many times as many as in other known contactors. The bodies must, of course, be mechanically strong enough to withstand such a large number of operations.

The active material is chosen partly for its electrical and partly for its mechanical properties. Thus this material must not give rise to over-voltages (by chopping small inductive currents) greater than are allowable in relation to the voltage for which the switch is to be used, and the material must also have a suflicient voltage interrupting ability for the particular switch required: both these properties are related to the arclng voltage of the material (which in turn is related in general to the thermal conductivity of the material and its volatility or boiling point. Since the thermal con- :ductivities of a large number of active materials are of the same order, the differences between different active materials depend to a large extent on their different volatilities). In addition, the material must not form an inadherent film (such as would be found if carbon were employed), for the film would be disintegrated and removed by the operation of the switch. Mercury too is not suitable, partly because of its natural fluid state which makes it difficult to retain in the recesses of both contacts, and partly because its great volatility would quickly lead to its being lost from the recesses by evaporation. It has been found that to interrupt voltages of the order of 400 volts, the active material may be arsenic, bismuth or an amalgam of mercury (the bodies ,of the contacts being made of copper, steel, molybdenum, tungsten or a sintered mixture of copper and tungsten); to interrupt voltages of the order of 3,300 volts, lead, cadmium, sodium or zinc may constitute the active ma terial (the bodies then being of copper, steel, molybdenum, tungsten, aluminium or a sintered mixture of copper and tungsten); for voltages of the order 6,600 volts, the active material may be magnesium or tin (the bodies then being of copper, steel, molybdenum, tungsten or a sintered mixture of copper and tungsten); for voltages of the order of 11,000 volts the active material may be aluminium or silver (the bodies then being of steel, molybdenum or tungsten); and for voltages of the order of 22,000 volts, the active material may be copper (the bodies again being of steel, molybdenum or tungsten). The material of the bodies mentioned above are all of a more refractory nature than the active material employed with them as well as being of a higher arcing voltage.

The vacuum in which the switch operates should not have a pressure higher than 10- millimeters of mercury and for voltages above 11,000 volts, this pressure should be less than 10- millimeters of mercury. Rigorous degassing of the'switch is, of course, essential: in particular active materials having high vapor pressures should be vacuum distilled.

In order that the invention may be more easily understood and readily carried into effect one example of a switch constructed according to it and suitable for use as a contactor, together with a modified form of contact, will now be described with reference to the accompanying drawings in which:

Figure l is a side elevation, partly in section of the switch;

Figure 2 is a plan View of the lower contact of the switch;

Figure 3 is a cross-section of this contact, taken along the line IIIIII of Figure 2; and

Figure 4 is a corresponding cross-section of the modified form of contact.

Referring first of all to Figure 1, the envelope in which the switch is housed includes a glass tube 1, two nickeliron alloy tubes 2 and 3 fixed to opposite ends of the tube 1 and a pair of metal end fittings 4 and 5 which are brazed on to the tubes 2 and 3 respectively. The upper fitting 4 is formed with an opening through which a copper stem 6, threaded at both ends, passes into the envelope. This stem 6 is brazed onto the fitting 4, as indicated at 7. A conductor 8 for the alternating current is secured to the stem 6 by nuts 9.

The lower fitting 5 is secured by screws 10 to a mounting bracket 11 which is formed with an opening in which is brazed a tube 13 for guiding a reciprocatory copper stem 14. This stem 14 is threaded at both its ends and is reciprocated by conventional mechanism (not shown) through an insulating sleeve 12. The other conductor for 4 alternating current, shown at 15, is secured to the stem 14 by nuts 16.

In order to make the envelope airtight, a metal bellows 18 is brazed at one end to the fitting 5 and at its other end to the stem 14. A bellows shield 19, screwed onto the fitting 5, and extending around the bellows 18 and fitting closely around the stem 14, serves to protect the bellows during vacuum processing. This vacuum is established through a tube 20 which is then sealed off.

The operating voltage of the contactor depends, of course, on the sizes and the materials of the contacts, which are identical and shown at 25a and 25b. The particular contactor being described is to be employed for frequent interruption of currents up to 300 amperes and for occasional interruption of up to 3000 amperes, and for this purpose the contacts 25 are of cylindrical shape having a diameter of one and one eighth inches. The contactor is to be employed at 3,300 volts, and thus each contact is made up of a mechanically strong steel body 26 (see Figure 3) having in its operating face 28 a number of recesses 29 each containing cadmium, indicated at 30. Each contact 25 is formed with a threaded recess 27 in its rear for screwing onto the stem 6 or 14, and each is retained in position by a split pin 27a.

As previously indicated, the major part of the cadmium moves back and borth between opposite recesses as the switch is operated, but a little is deposited on the operating face 28 at 28a to form a film which finally attains a stable thickness. An extremely small proportion of the cadmium passes out of the region of the operating faces and collects on the inner surface of a metal ring 31 (see Figure l) which is held in place at the level of the contacts by dimples 32 formed in the glass tube 1. The amount of cadmium lost in this way is exceedingly small partly because the contacts 25 are, never separated by more than a millimeter or so.

In the modified form of contact shown in Figure 4, only one centrally arranged recess 40 is provided.

The recesses may of course be in the form of grooves.

While the invention is described with reference to a switch suitable for use as a contactor it may also be applied to switches suitable for use as circuit breakers.

I claim:

1. A vacuum switch comprising an evacuated envelope, and two contacts mounted in said envelope for movement into and out of engagement with one another to close and open said switch, each of said contacts comprising a mechanically strong body having at least one recess formed within its contact area, and non-fluid conducting material disposed in said recess, the contact bodies being arranged to engage each other in a contact area completely surrounding said recess and exerting pressure on one another through their operating faces on closing of the switch, said contact bodies being formed of a material substantially more refractory than said nonfluid material, and said non-fiuid material having a lower arcing voltage than that of the said bodies and being capable of forming an adherent film on the operating faces of the contact bodies during operations of the switch.

2. A switch according to claim 1, each of the contact bodies having at least two recesses, each recess on a contact being arranged directly opposite a recess in the other contact.

3. A switch according to claim 1 for use at 400 volts, said non-fluid material being chosen from the group of relatively low arcing voltage materials consisting of arsenic, bismuth and amalgams of mercury, and the contact body being made of a material chosen from the group of relatively high arcing voltage materials consisting of copper, steel, molybdenum, tungsten and a sintered mixture of copper and tungsten.

4. A switch according to claim 1 for use at 3,300 volts, said non-fluid material being chosen from the group of relatively low arcing voltage materials consisting of lead.

cadmium, sodium and zinc, and the contact body being made of a material chosen from the group of relatively high arcing voltage materials consisting of copper, steel, molybdenum, aluminium, tungsten and a sintered mixture of copper and tungsten.

5. A switch according to claim 1 for use at 6,600 volts, said non-fluid material being chosen from the group of relatively low arcing voltage materials consisting of magnesium and tin, and the contact body being made of a material chosen from the group of relatively high arcing voltage materials consisting of copper, steel, molybdenum, tungsten and a sintered mixture of copper and tungsten.

6. A switch according to claim 1 for use at 11,000 volts, said non-fluid material being chosen from the group of relatively low arcing voltage materials consisting of aluminium and silver, and the contact body being made of a material chosen from the group of relatively high arcing voltage materials consisting of steel, molybdenum and tungsten.

7. A switch according to claim 1 for use at 22,000 volts, said non-fluid material being copper, and the contact body being made of a material chosen from the group consisting of steel, molybdenum and tungsten.

8. A switch according to claim 2, wherein said two contacts are identical and are arranged directly opposite one another.

References Cited in the file of this patent UNITED STATES PATENTS 550,360 Wurts Nov. 26, 1895 1,556,573 Andersen Oct. 13, 1925 1,648,100 Aichele Nov. 8, 1927 1,875,765 Scherbius Sept. 6, 1932 1,906,602 Hull May 2, 1933 2,064,998 Waite Dec. 22, 1936 2,234,834 Scott Mar. 11, 1936 2,253,401 Slepian Aug. 19, 1941 2,294,783 Ely Sept. 1, 1942 2,370,400 Graves Feb. 27, 1945 2,641,670 Graves June 9, 1953 2,794,885 Jennings June 4, 1957 FOREIGN PATENTS 289,021 Great Britain June 26, 1929 591,183 Great Britain Aug. 11, 1947 

